Titanium metal combines the strength of steel with the light weight of aluminum, and for this reason the metal and its alloys are used extensively in the aerospace and aviation and high performance sports equipment industries. Titanium metal and its alloys (hereinafter, “titanium materials”) are also resistant to corrosion under ambient conditions. Because of its mechanical properties and chemical resistance to degradation by bodily fluids, titanium materials are used extensively to fashion metal fitments used as dental and orthopedic implants (medical implants). When a metal fitment is used as a medical implant, it is important to create a stable bond between bone tissue and the surface of the implant. Poor bonding at the interface between the surface of the implant and the bone tissue leads to low mechanical strength of the bone-to-implant junction and the possibility of subsequent implant failure.
An important goal for interface optimization is to use species which are biocompatible and which enable bone mineralization at the interface following implantation. Bone tissue is a combination of protein and mineral content, with the mineral content being in the form of hydroxyapatite. Currently, there is no effective way to obtain strong attachment of incipient bone with the implant material at the interface between the surfaces of the two materials in order to “stabilize” the implant.
The problem of synthesizing an adhesion-promoting interface on implants is often approached from the prospective of high temperature methods, including using plasma or laser-induced coating techniques. However, these methods engender problems of implant heating and surface coverage. For example, calcium phosphate deposition at high temperatures can give rise to ion migration. Plasma-induced phosphate coating of a titanium substrate gives surface hydroxyapatite as well as surface calcium phosphate, titanates and zirconates. Therefore, control of surface stoichiometry can be problematic, and defects at the interface may translate into poor mechanical strength.
The use of intermediate layers, for example of zirconium dioxide, to enhance hydroxyapatite adhesion and interface mechanical strength has been explored with success. However, a practical limitation involving laser or plasma deposition is that it is hard to obtain comprehensive coverage on a titanium implant of complex 3-dimensional structure. The zirconium dioxide interface formed at high temperatures is of low surface area and maintains few, if any, reactive functional groups for further surface modification chemistry. A silicate, phosphate or phosphonate interface can function to nucleate the growth of hydroxyapatite, thereby minimizing implant failure and the attendant need for serial revision implant surgery, which can be a consequence of unstable implant-to-bone interaction, but, as described below, there is little success of providing such surfaces on the oxide surfaces of titanium materials.
Solution-phase surface processing does not suffer from the practical limitations of surface coverage that can be attendant with plasma or laser-based deposition methods, and procedures involving formation of hydroxyapatite from solution, often using sol-gel type processing, have been accomplished. Elegant methodologies have been developed in which graded interfaces have been prepared, extending from the pure implant metal to the biomaterial at the outer extremity by way of silicates. However, while solution-based procedures are inexpensive and give rise to materials resistant to dissolution by bodily fluids, adhesion of the hydroxyapatite to the implant metal is less strong than is observed when deposition is accomplished by plasma spraying techniques.
The deficiency of these solution approaches may lie in the nature of the native oxide surface of titanium materials. Exposure of a clean surface of titanium materials to oxygen results in the spontaneous formation of surface titanium oxides (native oxide). The exact chemical stoichiometry and structure of these oxides varies from material to material, and with depth in the oxide layer, with environmental variables, and with the processing history of the material. The oxide layer may be stoichiometric, super-stoichiometric, or sub-stoichiometric with respect to TiO2, a stable oxide of titanium. Generally, the uppermost layer of the native oxides comprises some form of TiO2. It may be crystalline, but if crystalline, it is generally disordered. Typically, many different phases exist within the oxide layer between the metal and the ambient environment. Generally the uppermost oxide layer includes widely dispersed hydroxyl functional groups bonded to a titanium atom. The surface forms spontaneously by exposing the metal or alloy to the ambient environment, and is alternatively described as the “native oxide surface” of a titanium material.
The inert nature of a native titanium oxide surface, even with regard to bonding with typical adhesives, is known and has been studied, for example, by Srivastava as reported in the Intern. J. Adhesion and Adhesives, 23, 59-67 (2003). In this study, a film of an adhesive composition, for example, an epoxy-based adhesive applied to the freshly cleaned surface of a Ti-6Al-4V titanium alloy coupon was observed to oxidatively degrade at the metal/native oxide/proxy coating interface resulting in a reduction of adhesive strength between the metal and the adhesive coating.
This oxide layer protects the underlying material from further chemical attack in that it is generally resistant to further chemical reaction. This property of the oxide surface of titanium materials mitigates against the adhesion of other moieties to it, for example, coatings and bone tissue, making application of an adhesion-promoting coating to the native oxide surface problematic. For example, a phosphorous-based acid incorporating into its structure an organic moiety, such as organophosphonic acid, does not readily form an adherent coating with a bulk metal under ambient conditions. This is in contrast to other metals that possess oxide coatings such as tin, iron, aluminum and copper, or their alloys, for example, steel, or bulk oxides, for example, mica, all of which yield an adherent film when treated with such acids. Film formation of the type observed between, for example, zirconium surface oxides and organophosphonic acids, is not observed with the oxide surface of titanium materials. An example of this is described by Gao et al., Langmuir, 12, 6429 (1996).
For many materials other than titanium, as described above, the native oxide surface can be altered chemically (derivatized) by exposing the surface to various hydrolytically reactive reagents. An example of derivatizing an oxide surface in this manner is exposure of the surface of a silicate (silicon oxide) to a trimethylsilyl derivative which “silanates” the surface, converting it from a hydrophilic surface to one that is hydrophobic by bonding trimethyl silyl functional groups to the native oxide surface. Typically, chemically derivatizing a surface is the most cost-effective method to achieve uniform surface coverage on surfaces having complex shapes, and in general represents the least impact to the mechanical properties of the material derivatized compared to other surface modification techniques.
Silanization has long been considered the benchmark method for attaching organics to titanium and its alloys, for example, Ti-6Al-4V, via their native oxide coatings. Direct silane-surface bonding is limited by surface hydroxyl group content, and the OH group content of the Ti native oxide surface is low, accounting for only about 15% of total surface oxygen. Low yields of direct surface silanization result, and silane reagent crosslinking can be the dominant mode of reactivity. Unfortunately, surface-bound and cross-linked siloxanes can be hydrolytically unstable, which can further reduce amounts of key organics that are coupled to simple surface silanization reagents under aqueous conditions.
For metal oxide surfaces which are not derived from titanium metal or its alloys, for example, the oxide surface of aluminum and its alloys or the oxide surface of silicon, it is common to carry out reactions between organometallic complexes and hydroxyl groups terminating the oxide layer to provide for a surface that is amenable to further derivatization or passivation. For these “non-titanium material” oxide surfaces, in cases in which the natural occurrence of hydroxyl groups per unit area of surface is too sparse to provide for the formation of a dense surface layer of derivatization products, it is known to subject the oxide surface to a variety of hydrothermal treatment schemes to increase the density of hydroxyl sites and improve the reactivity of the surface toward derivatization. It will also be appreciated that the methods and coating layers of the present invention, described below for titanium native oxide surfaces can be equally well applied to the native oxide surfaces of other materials.
In general, the coverage of naturally occurring hydroxyl sites which form on titanium material oxide surfaces is too sparse to provide dense-coverage layers of derivatization product on the surface of the material by traditional derivatization routes. Additionally, attempts to increase the actual density (sites/unit area) of hydroxyl sites on the surface oxides of titanium materials results in “roughening” of the surface, which, while increasing the number of hydroxyl sites on the projected (nominal) surface area of the bulk material, also increases the actual surface area of the material, and, consequently the density of hydroxyl sites/unit area remains approximately constant.
If the protective oxide layer of a titanium material is dissolved under conditions in which its re-formation is inhibited, rapid corrosion of the material will occur, an example of which is reported by Fang et al., Corrosion, 47, 169 (1991). Reducing acids, for example hydrobromic, sulfuric, and phosphoric acid, under the proper conditions of heat and acid concentration, can attack titanium metal and its alloys. Such attack is especially facile when oxidizing agents, for example, air, are excluded from the surface of the material under attack. For example, titanium metal rapidly dissolves in 85% phosphoric acid at 80-100° C. yielding a solution from which titanium(III) dihydrogen orthophosphate (Ti[H2PO4]3), (“Ti-phosphate”) can be isolated.
A need exists for a methodology that combines the benefits such as those obtained from the physical deposition of interfacial zirconium dioxide with the coverage, processing and speciation control of solution-based methods described above. Additionally, a need exist for a methodology that makes the oxide surface of titanium materials amenable to solution process derivatization that establishes dense-coverage, robust and adherent coatings amenable to establishing a strong interface between the oxide surface and materials with which the surface is placed in contact, for example, the provision of a surface which exhibits strong bonding to an adhesive film applied to it, the promotion of adhesion between medical implants made of titanium materials and the tissues into which they are implanted, and the provision of a surface which promotes bone integration into the implant/tissue interface.